W ant to understand bilateral symmetry? Stand naked in front of the mirror and imagine a line extending from your forehead down the center of your nose, through your navel, and between your legs. Right and left eyebrows, eyes, cheekbones, ears, shoulders, elbows, hips, thighs, knees, and feet match perfectly. Right? Well, sort of, says John T. Manning, an evolutionary biologist at the University of Liverpool in England. Despite nature's best efforts, right never exactly matches left. As researchers are finding out, the degree of mismatch can communicate potential shortcomings. New work suggests that some species, including humans, pay attention to these minor inconsistencies and may even pick sexual partners based on them and not just because we have embraced Madison Avenue standards of beauty (part 2, next week). Nature uses symmetry to signal the well-being of an individual, says Randy Thornhill, an evolutionary biologist at the University of New Mexico in Albuquerque. He and about a dozen other researchers have recently taken a hard look at small deviations in symmetry: Is one side of the head slightly wider than the other? kneecap a little broader? An ear a wee bit smaller? With steel calipers and multiple measurements, they are documenting subtle differences between right and left features in many kinds of organisms. These differences add up to an individual's socalled fluctuating asymmetry fluctuating because there is no consistent trend for one side to be a particular way relative to the other, only for features to vary, Manning explains. These fluctuations represent one of three kinds of deviations from nature's original design. Rightor left-handedness in animals exemplifies another kind, called antisymmetry. The third, directional asymmetry, results when development occurs on one side of the body but not the other, as in the case of the mammalian heart. The aberrations involved in fluctuating asymmetry are more random than these other two and can arise both during development and over the course of an animal's lifetime. These mismatches seem to indicate well-being in a very broad sense. It's really a measure of the developmental stability, Manning notes. Such stability reflects how well an individual's genes manifest themselves under various environmental conditions, especially stressful ones. If an organism's genetic makeup translates into a healthy, symmetrical individual even when confronted with disease, starvation, abnormal temperature, pollution, or parasites, then offspring should also prosper and that's appealing to any creature's survival instincts. [Fluctuating asymmetry] potentially provides information about the interface between the environment and the genotype, agrees Therese A. Markow, a biologist at Arizona State University in Tempe. Nevertheless, she and others worry that some scientists are going too far too fast in promoting symmetry as a fitness indicator and, consequently, as a determinant of mate choice. One shouldn't be blinded by the simplicity of symmetry, says Craig Packer of the University of Minnesota in C,. ch